Refrigeration systems, such as those used in screw compression technology generally have extra cooling capacity during most operating modes, thereby leading to inefficient system operation. Based upon the specific mode in which the compressor system is operating, different refrigeration capacity controls provide for desired performance in the most efficient manner. Electronically controlled suction-line-modulation valve is one such means and is often included in system configurations. For the heating and defrost operations in a typical refrigeration system which includes a compressor, a condenser, a thermal expansion valve and an evaporator, the capacity control strategy doesn't have a lot of flexibility and therefore is not very efficient. In standard heating/defrost cycles, the temperature difference between the air being heated and refrigeration stream is very limited due to the fixed temperature of the air within the enclosure and limited compressor pumping capacity. Accordingly, improving the temperature differential in some manner can dramatically improve the heating or defrost cycle efficiency and improve overall system performance.
The prior art does include several manually adjustable preset valve designs for controlling system capacity or throttling the refrigerant stream between the evaporator and compressor. These valves include suction service valves and other manually adjustable valves, such as a compressor-crankcase-pressure regulating (CPR) valves. However, the drawback of these valves is that any capacity adjustments made therewith must be done manually, thereby requiring constant attention to the mode of the refrigeration cycle. These valves are almost impossible to control due to a variety of operating conditions and modes as well as transient system behavior. Accordingly, CPR valves are manually preset for a specific condition and operating mode, without changing state.
The prior art includes a plurality of refrigeration systems, some of which use valves positioned between the evaporator exit and the compressor inlet, on the suction line. Some of these valves are used to control system capacity but none are electronically controlled and programmable to control system heating/defrost capacity in conjunction with thermal expansion valve adjustments. For example, U.S. Pat. No. 4,977,751 discloses a valve system having a modulation valve which also performs the function of a compressor throttling valve. The valve is positioned between the evaporator outlet and compressor inlet, as represented in FIG. 2 by valve 54, evaporator 42 and compressor 14. The modulation valve 54 controls refrigerant flow to compressor 14. A load circuit operates valve 54 to perform the function of a throttling valve when load reduction is required in the cooling mode. An overload condition of a compressor prime mover overrides a control and selects a predetermined load control position of the valve on overload. The timer switches back to the control when a predetermined recovery time has past. In heating and defrost, the system automatically selects the load control position of the valve 54 for the duration of the mode, as does the ambient air temperature sensor when the ambient precedes a predetermined value. While the modulation valve 54 is controllable during the valve cycle, during defrost and heating, the valve is only controllable to a preset position or opening size. Variations in the cycles in accordance with thermal expansion valve activity cannot be accounted for to achieve optimum system capacity. Accordingly, while controllable, the valve 54 of this system lacks flexibility otherwise desirable in the defrost and heating modes, and in conjunction with the thermal expansion valve.
Additional patents drawn to systems used to vary system capacity during cooling are shown in U.S. Pat. No. 4,689,967 to Han et al, and for U.S. Pat. No. 4,712,383 to Howland et al, and U.S. Pat. No. 4,742,689 to Lowes. None of the systems disclosed in these patents exhibit the controllability of the defrost and heating modes and the hot-gas valve, which is desired to have the complete degree of capacity control and reach optimum system heating/defrosting capability.
There exists a need, therefore, for a refrigeration air conditioning cycle having an electronically controlled Suction Modulation Valve (SMV) which is operable to control system capacity in the heating and defrost modes.